1. Field of the Invention
The present invention relates to a recording and reproducing apparatus which is preferably used in a magnetic recording and reproducing apparatus of rotary head type such as a so-called DAT (digital audio tape recorder).
2. Description of the Background Art
FIG. 1 is a view showing a running mechanism of a magnetic tape 1 in a magnetic recording and reproducing apparatus of rotary head type. FIG. 1 (1) is a view showing a state in which a cassette 2 is loaded in the magnetic recording and reproducing apparatus of rotary head type. The magnetic tape 1 is wound on both a supply side reel 3a and a winding side reel 3b, respectively. The running mechanism includes and consists of a post 4, a tension post 5, guide rollers 6a and 6b, inclination posts 7a and 7b, a rotary drum 8, a pinch roller 9 and a capstan 10.
The post 4 and the tension post 5 are provided to keep the tension of the magnetic tape 1 applied to the rotary drum 8 constant. The rotary drum 8 is rotated in the direction of an arrow 11, centering around the rotary shaft 8a. The magnetic heads 12a and 12b are mounted at a position where they are opposite to each other by 180.degree. in the rotary drum. The magnetic heads 12a and 12b have different azimuth angles, for instance an azimuth angle of +20.degree. for the magnetic head 12a and an azimuth angle of -20.degree. for the magnetic head 12b, in order to prevent noises due to cross talk. The rotary drum 8 is so installed on the rotary shaft 8a to be inclined by the angle .theta..degree. from an axis y on the plane x-y as shown in FIG. 1 (2).
The magnetic tape 1 is wound on the rotary drum 8 with a winding angle of about 90.degree. by means of the guide rollers 6a and 6b and the inclination posts 7a and 7b. When recording data on the magnetic tape 1, the magnetic tape 1 runs in the direction of an arrow 13 by means of the pinch roller 9 and the capstan 10, and the rotary drum 8 is rotated in the direction of an arrow 11. Thus, the magnetic heads 12a and 12b trace the surface of the magnetic tape 1 alternately, and data is recorded in such track patterns as shown in FIG. 2.
Tracks a1, a2, a3, a4, . . . are portions recorded by the magnetic head 12a on the magnetic tape 1, and tracks a1, a2, a3, a4, . . . are read out by the corresponding magnetic head 12a having the same azimuth angle. Tracks b1, b2, b3, b4, . . . are portions recorded by the magnetic head 12b and are read out by a corresponsing magnetic head 12b. A track ai (i=1, 2, 3, 4, . . . ) and another track bi which are usually a pair constitute a frame fi. Data to be recorded is recorded with a frame number, which is distinguishing information, and provided for every frame. The frame number is a sequential number which is increased one by one per frame during recording.
In the case data of a computer is recorded on the magnetic tape 1 by using such a magnetic recording and reproducing apparatus of rotary head type as shown in the above description, it will become necessary to rewrite data after a specified frame. For example, in the aforementioned FIG. 2, it is assumed that data following frame f2 is rewritten. Reproduction is carried out up to the frame f2, and the mode is changed over from reproduction to recording at the point when reproduction is terminated at the frame f2, and a so-called relaying is performed. At this time, as the control system providing tracking control for reproduction is different from that for recording, a minute difference may occur in the running speed of the magnetic tape 1. Also, only by changing over the magnetic heads 12a and 12b from a mode of reproduction to another mode of recording, will the magnetic head 12a scan the position denoted by reference symbol P1 in FIG. 2 in the direction of arrow R1 wherein new data is recorded in the area 15 shown with oblique lines. Therefore, as shown in FIG. 3 (1), there is a problem that a part of track b2 which constitutes the frame f2 will be destroyed by the track A3 which constitutes the frame F3. Here, tracks A3, A4, . . . and tracks B3, B4, . . . are those whose data has been already written, and a track Ai (i=3, 4, . . . ) and a track Bi constitute a frame Fi.
In order to solve the above problem, in the case recording is attempted to be started accurately from the frame f3, very complicated circuits are required. For instance, if reproduction is carried out with the tracking control of the magnetic heads 12a and 12b slid and reproduction is changed over to recording at the point when the reproduction of the frame f2 is terminated, it is possible to establish tracking patterns of good form. For this reason, a control circuit for accurately controlling the timing of change-over from reproduction to recording is needed, thereby resulting in circuits of very complicated structure.
Also, if the recording mode is changed over from the reproduction mode at the point when reproduction is conducted up to the frame f3 and reproduction of the frame f3 is terminated, another problem will result. That is, the track b2 which constitutes the frame f2 is not destroyed, but the frame f3 before rewriting may remain, and data in the frame f3 will be reproduced during reproduction.